Neurotrophins are a family of conserved growth factors that play critical roles in the differentiation of immature neurons and in the survival of their mature counterparts. All neurotrophins are synthesized as dimeric precursors (i.e., proneurotrophins) which are cleaved and released as smaller, C-terminal mature forms. Mature neurotrophins mediate their effects by interacting with two structurally unrelated receptors: p75NTR, a member of the tumor necrosis factor (TNF) receptor superfamily, and the Trk receptor tyrosine kinases. In addition, ligand activation of p75NTR can initiate apoptosis. Resolution to the paradox of neurotrophins mediating both pro-survival and pro-death outcomes has recently been gleaned by the findings that proNGF and proBDNF selectively bind to p75NTR to elicit apoptosis, whereas mature NGF and BDNF activate their cognate Trk receptors to mediate survival. Accordingly a third receptor, sortilin, that selectively binds to the pro-domains of proNGF and proBDNF, forms a co-receptor complex with p75NTR to convey apoptotic signaling. Amongst the neurotrophin members, neurotrophin-3 (NT-3) displays the highest levels of expression within the nervous system and is the most widely distributed neurotrophin in non- neuronal target tissues. Yet little is known of whether the pro-form of NT-3 is pro-apoptotic and if so, the physiological significance of proNT-3 release during development and in adulthood. Preliminary analysis from my lab revealed that proNT-3 can be secreted from primary neurons, and when compared with other proneurotrophins, proNT-3 is equally effective in eliciting apoptosis. This proposal will thus focus on the roles of proNT-3, its co-receptor interactions, and post-receptor signaling that ultimately regulate distinct biological outcomes. These in vitro approaches will be extended to in vivo models to test three inter-related hypotheses. Specifically, I plan to (i) characterize proNT-3:receptor interaction that modulates its biological activity, (ii) define the underlying mechanism that mediates proNT-3-induced cell death, and (iii) understand the actions of proNT-3 in the nervous system by in vivo analysis of gene replaced animals. Given the widespread expression of NT-3 in and outside of the nervous system, these studies will lay the necessary foundation for the clinical treatment of birth defects, neurodegenerative disorders, spinal cord injury and mental illness.